Omaha, Nebraska, in particular saw 5.3 inches of rain, a one-day record for June precipitation. That caused flooding, knocked out power, and even halted vote-counting in the primary elections in Council Bluffs. Images from the storm showed extensive hail damage on vehicles in the area.

The storm now is moving to the East and South, and conditions are ripe for it to turn into a severe "derecho" storm. Derechos — meaning "straight" in Spanish — are wide swaths of thunderstorms that create high-powered winds in a straight line, as opposed to the swirling circular winds of tornadoes. As The Weather Channel explains, "the impact is somewhat like that of a landfalling hurricane," giving the storm its alternate name. "As we like to say, it doesn't have to rotate to be dangerous," Bill Bunting, an Oklahoma-based forecast operations chief at the Storm Prediction Center, told the AP.

As of early Wednesday morning, the storm stretched from Missouri to Ohio, according to Accuweather.

The area that the storm threatens includes 15 million people in all. As the storm moves southeast over Wednesday, the Tennessee Valley and upper Mississippi Valley should expect high winds and more rain.

News reports are focusing on the Germanwings pilot's possible depression, following a familiar script in the wake of mass killings. But the evidence shows violence is extremely rare among the mentally ill.